|The diagram is a simplified
representation of the actual contents of the device.
The first thing to note is the 3 resistors between pin 8 and 1 (all
Using the voltage division theorem you can calculate that the top resistor
will drop !/3 rd of the supply - there will be 2/3 of the supply to ground.
The bottom resistor will drop 1/3 rd of the supply to ground
Two comparitors are connected to this resistor network
The top comparitor inputs connect pin 6 (THRESHOLD) and the top resistor
of the network. (point A)
The bottom comparitor inputs connect pin 2 (TRIGGER) and the bottom
resistor of the network. (point B)
Once pin 6 (THRESHOLD) voltage tries to rise more than 2/3 of
the supply (rising voltage caused by capacitor C charging via resistor
R1 and R2) this comparitor will change state
The flip flop will trigger in the 555 timer which will result in the
transistor connected to pin 7 (the discharge pin) turning on.
When this fires the external capacitor will discharge until the voltage
reaches 1/3 rd of the supply.
At this point the circuit will automatically trigger itself because
the bottom comparitor will be triggered by the falling voltage.
The capacitor will now start to charge and the sequence will repeat
Remember - Astable pins 2 and 6 are connected together to supply
via a resistor
In the monstable the output is permanently being pulled low
The trigger that is applied to the trigger pin (pin 2) also pulls the
pin towards ground.
This makes the lower comparitor change state triggering flip flop
This turns off the transistor that is permanantley on in the monostable
This allows the capacitor to charge.
Once this reaches 2/3 rds the supply voltage then the top comparitor
will trigger, the discharge transistor will turn on once again discharging
It will remain in this state until the the trigger switch is pressed